Usually, photos taken on the Moon or the International Space Station don’t show stars—only a deep blackness occupies the background of the images. The explanation for this is relatively simple to understand, but requires some knowledge of the principles of photography and the behavior of light.
Is it possible to see stars in space?
Before explaining why we don’t see stars in these photos, know that yes, it is possible to see stars in the dark in space. They don’t appear in most photographs, but they are visible to the eyes of astronauts – and in much greater numbers than we see here on the surface, where light pollution prevents the appearance of many stars in the sky.
See this beautiful record of astronaut Jack Fischer, who was on the ISS in 2017. He made a point of posting this time-lapse to answer questions from followers about the possibility of seeing stars on the ISS, or not.
Where are the stars in the ISS photos?
Here on Earth, the atmosphere causes the sun’s rays to scatter. Therefore, during the day the sky is blue and the light takes care of everything. You might think that in space, since there is no atmosphere to scatter sunlight, the “sky” is always dark, even at midday. And you’re right — up to this point.
It’s just that the fact that the “space sky” is dark even during the day does not mean that it is not receiving sunlight. In fact, an astronaut looking out the windows of the ISS at daytime receives a lot of direct sunlight, as much (or even more) than we receive on a summer day on the surface.
From here come the knowledge of photography. Imagine that you are in a dimly lit house, in front of a window, and it is a cracking sun outside. You want to photograph the landscape you see through the window, bright and colorful. It will be necessary to use a fast exposure and a very narrow aperture so that the click is fast and accurate, capturing the outside light in the right measure, without bursting.
And the principle of photographing great contrasts of light in space is the same. After all, sunlight above Earth illuminates everything as much as it does down here.
To take beautiful pictures of the Earth, the Moon, the metallic structure of the ISS or astronauts’ suits in space, astronauts need a fast exposure with a narrow aperture. Thus, a minimum amount of light reaches the camera sensors, enough to capture the image of the desired surface, person or object, which will be brighter than the rest of the scene.
Ok, but why don’t the stars appear in the background of the photo, since they stand out so much amid the dark space? It’s just that fast exposure captures the light of what’s very bright in front of you — and that’s it.
To record more distant and weaker glows, the sensor would need to stay open longer, capturing more light. But that would ruin the rest of the image, which would be overexposed. And that’s why astronauts appear without a single star around them in photos taken in space.
The photo below, made with long exposure, was released by NASA precisely to illustrate this issue. In it, we see the Earth somewhat blurred and with a half-blown glow, while the space in the background brings thousands of bright stars. The exposure time was long enough to capture the stars, but too long not to blow out Earth’s light.
In short: when the camera is set to quickly pick up light from bright objects in the foreground, there isn’t enough time for the sensor to also register the fainter glow coming in the background. And longer clicks, despite capturing the starlight in the background, spoil the rest of the image with loss of focus and burst brightness.
And on the moon, can you see stars?
The above explanation also applies to the photos taken on the Moon, which show landscapes, ships and astronauts clearly, but in contrast to a dark background and without a single star.
The lunar soil is highly reflective, which requires fast and tight camera settings. Otherwise, the photo will burst where there is more light. However, this prevents the sensor from capturing the brightness of the stars in the background.
Here on Earth, the rule is the same: to photograph stars in the night sky, you need to combine long exposures with wider apertures. Thus, a greater amount of light enters the sensor, enough for it to register the distant stars. However, if any minimally reflective object or surface is in this same framing, all the reflected light will be registered in the image, generating an unwanted luminous “burst”.
If you found this whole theory too complicated, here’s the tip in the video below, in which the Sidereal Messenger goes deeper into this issue. To make it easier, he exemplifies in a practical and visual way all the concepts explained here!
